SCIENTIFIC NEAA/S. 



[June 1st, if 



receiver, till drops were excreted. Removing the receiver, 

 he selected a blade with a drop attached to it. Over this he 

 placed a small dry glass receiver, so as to isolate the tip 

 from the damp air of the large receiver. Its open end was 

 closed by a thin plate of metal ; through a small slit in this 

 plate the tip of the blade was entered ; and to prevent 

 moisture entering and coming in contact with this tip, an 

 air-tight joint was made between the blade and the metal 

 with india-rubber solution. The large receiver was then 

 placed over the turf to prevent evaporation from the lower 

 part of the blade. After some hours, a drop was found on 

 the tip of the blade inside the small receiver, of the same 

 size as the drops on the turf blades outside the small 

 receiver. This showed that these drops were really 

 exuded by the plant, and not extracted from the air. 

 When the roots were cut, no drops were found. But when 

 to the rootless stems the india-rubber tube, with water 

 pressure, was applied, drops were immediately seen at the 

 tips of the blades. 



Careful observation showed him that the formation of 

 drops on plants that exude moisture depends on the rate of 

 supply, the humidity of the air, and the velocity of the 

 wind. The drops that first make their appearance on grass 

 are not drops of dew at all, and their appearance depends 

 not on the laws of dew, but on those of vegetation. Hence 

 the varied distribution of moisture on plants and shrubs on 

 dewry nights. Much of the moisture that collects on plants 

 at night does not form like dew on dead matter. Dead 

 matter gets equally wet where equally exposed, and the 

 moisture does not collect on it in isolated drops, as it does 

 on plants. The drops which appear on grass on clear nights 

 are not dew at all, and they make their appearance on sur- 

 faces that are not cooled to the dew-point. That is false 

 dew. But after these drops have been forming for some 

 time, true dew makes its appearance when the radiation in- 

 creases, and the plants get wet all over as in the case of 

 dead matter. 



True dew is formed on fewer nights than we might at 

 first imagine. On many nights on which grass gets wet, no 

 true dew is deposited on it ; and on all nights, when vege- 

 tation is active, the exuded drops always make their 

 appearance before the true dew. When, therefore, we 

 walk over the lawn in early evening, the wet which we 

 may brush off with our feet is not dew, but the juices exuded 

 through the veins of the grass blades. Careful observation 

 will show the difference between the exuded juice-drops 

 and the true dew ; the moisture exuded by grass in a drop 

 of some size is always excreted at a point situated near the 

 tip of the blade and at the extremity of a vein, often when 

 the rest of the blade is dry, whereas true dew collects 

 evenly in a pearly film over all the blades. The false dew 

 appears in large glistening drops ; the true dew coats the 

 blade with a fine filmy lustre. 



This accounts for the strange phenomena often observed 

 in garden plants. Certain kinds are found covered with 

 moisture, while others are dry. Many plants of the Bmssica 

 family are heavily covered with diamond-like drops ; while 

 peas, beans, etc., growing beside them, are quite dry. 

 Again, in clusters of plants of the same kind, some are 

 found wet and others dry; besides, some branches are wet 

 while others are not so. According to the ages of the plants, 

 and their exuding powers, these phenomena present 

 themselves. On dead leaves no drops are found, although 

 there may be a thin pearly sprinkling of moisture all over. 

 The large drops, which we are accustomed, both in poetry 

 and science, to consider as dew, are not dew at all, but 

 watery juices from healthy plants; whereas the true dew 

 — not so often seen — is the fine film which is found on all 

 substances on a favourable night. 



SPEAKING APPARATUS FOR DIVERS. 



THE accompanying illustrations refer to a very simple 

 apparatus, which enables divers to hold viva voce 

 communication with their attendants, and it is equally 

 applicable to diving-bells or to helmets. The figure shows 

 a diver's helmet with the speaking apparatus {b) attached, 

 also a section of the latter which may be explained in few 

 words. A small hole is made in one side of the helmet, 

 and over this is fixed a thin metallic diaphragm {a), 

 suitably protected on the outside. This constitutes the 

 whole of the apparatus, and on being connected with 

 a flexible tube, leading to the attendants above water, 

 it can at once be used. Any one who has had occasion to 

 put on a diver's helmet must have noticed that the noise is 

 almost unbearable if the person with the helmet on speaks 

 to himself, even in a low voice. Incidentally, too, it 

 may be mentioned that a helmet conducts sound so well 



diver's helmet, with enlarged section of speaking 

 apparatus. 



that when two divers literally " put their heads together " 

 when under water, they can converse freely. Formerly 

 divers had to depend for communication with those above 

 water on signals conveyed by a line, and serious mistakes 

 sometimes occurred ; moreover, by this system the diver 

 could not report what he was doing, or say anything as to the 

 condition of his work. He either had to rise to the surface, 

 or write more or less illegibly on a slate. More recently 

 telephones have been used, but they are not so simple as 

 the apparatus now under notice, and with the latter no 

 battery is required. It might, perhaps, be suggested that 

 a mere speaking-tube without a diaphragm would do, but 

 there are two insuperable objections to this. In the first 

 place, if by mischance the tube became injured, water 

 would rush into the helmet with the most disastrous con- 

 sequences. Even if this did not occur, there would remain 

 the fact, that without a diaphragm the air pumped into the 

 helmet for breathing purposes would escape through the 

 speaking-tube, and it would then be impossible to use the 

 tube for speaking purposes. When the new system is applied 

 to a diving-bell, as shown in our illustration, the diaphragm 



